The invention pertains to the field of speed measuring devices and, in particular, to devices that measure the speed of a watercraft.
Venturi fluid flow measuring devices of circular or rectangular cross-section are well known in the art for the measurement of liquid in flow meters and gases in ducts.
Examples of circular and rectangular duct venturi devices are disclosed in a series of patents to Halmi including U.S. Pat. Nos. 3,686,946; 3,733,901; 3,733,902; 3,733,903 and 4,516,434 wherein he discloses configurations to minimize flow losses and minimize inaccuracies in measurement. U.S. Pat. No. 4,174,734 to Bradham also discloses a venturi flow meter having the upstream portion defined by a specific mathematical relationship based on a constant rate of acceleration of the fluid flow in the converging section of the venturi.
Venturis are also used for a variety of mixing devices such as carburetors wherein a liquid fuel is drawn into the venturi through a port and vaporized in a stream of air accelerated by the venturi. In the examples above noted the venturi forms a portion of a pipe or duct, round or polygonal in cross-section. The fluid, whether a gas or a liquid is blown or drawn through the pipe or duct by some external means such as a fan or pump. As a part of the duct or pipe the venturi does not move with respect to the duct or pipe.